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Old 9th May 2010, 07:32 PM   #1
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Default LED VU metre - how to achieve inertia.. ?

Hi guys, I am constructing a simple LED VU metre. The schematics includes a precision rectifier and a chain of comparators biased to the voltages for the respective levels in dB.. nothing special. But I have been reading recently about different metres and according to standarts the needle of a proper VU metre needs 300ms to go from -20 to 0 dB and another 300ms vice versa. But since LEDs are not that inert, i obviously need to insert a capacitor somewhere in the curcuit to achieve the desired inertia (so that it will take 300ms the '0-dB-LED' to illuminate after a 0 dB signal is fed at the input). Where to put that cap? How to calculate its value?
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Old 9th May 2010, 08:33 PM   #2
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Default http://www.electronickits.com/kit/complete/ligh/ck1005.htm

Stereo VU Meter
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Old 9th May 2010, 11:37 PM   #3
Irakli is offline Irakli  United States
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check this out:

http://www.national.com/ds/LM/LM3916.pdf

also this:

LED Audio VU Meter
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Old 10th May 2010, 09:02 AM   #4
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Well, thank you both, but I'm not asking about a schematic... I already have one. My question is specifically about how to achieve the desired rise and fall times of 300ms. Could someone help me, please?
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Old 10th May 2010, 04:44 PM   #5
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I found a peak detector circuit.
Using a 1458 op amp
V= 5 to 15 volts
Pin
1 to anode of D1 (1N914)
2 10k to cathode of D1
3 10k to input signal
4 -V
5 to cathode of D1
to R3 (other side of R3 to ground)(in parallel with C1)
to C1+ (1 to10ufd) (C1- to ground)
6 to pin 7 (2nd op amp is a voltage follower)
7 output
8 +V

Experiment with R3 to determine how slowly the capacitor drains.

You might try to make it more simple: no op-amp
Input to the cathode of a diode
Cathode of diode is output
R3 from output to ground
C1 from output to ground.

Last edited by RussellW; 10th May 2010 at 05:12 PM.
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Old 10th May 2010, 06:39 PM   #6
macboy is offline macboy  Canada
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Quote:
Originally Posted by ribolovec2 View Post
Well, thank you both, but I'm not asking about a schematic... I already have one. My question is specifically about how to achieve the desired rise and fall times of 300ms. Could someone help me, please?
If you had bothered to check out the references helpfully given to you, then you would have found that the National LM3916 datasheet has an Application Hint in it which appears to implement exactly what you are looking for. Check out figure 7, which implements a VU meter response circuit that "Reaches 99% level at 300 ms after applied toneburst and overshoots 1.2%".
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